How Much Concrete Do You Need Per Post Footing?

Under-ordering concrete is one of the most common — and most easily avoidable — hold-ups on a post-setting job. Run short mid-pour and you're stuck with a half-set footing while you wait for more bags. This guide walks through exactly how much you need, based on hole size and post depth, so you order the right number of bags the first time.

Concrete Volume by Hole Diameter and Depth

A post hole is roughly a cylinder, so the volume of concrete you need is based on the classic cylinder formula: volume = π × radius² × depth. In plain terms — a small increase in hole diameter has a much bigger effect on volume than the same increase in depth, because the radius is squared in the formula.

Here's what that looks like in practice for common hole sizes:

Hole diameter	Hole depth	Approx. volume
250mm	500mm	~25 litres
300mm	600mm	~42 litres
350mm	700mm	~67 litres

These are gross hole volumes — before subtracting the space the post itself takes up. A 300mm hole isn't dramatically larger than a 250mm one on paper, but the volume jump is significant, which is why it pays to measure your actual hole rather than assume a "standard" size covers every job.

Why diameter matters more than people expect

Because the formula squares the radius, doubling the hole diameter doesn't double the concrete volume — it roughly quadruples it. Go from a 250mm to a 350mm hole, less than a 50% increase in diameter, and the volume needed jumps by well over double. This is the single biggest reason people under-order: they eyeball the diameter increase as "a bit bigger" rather than doing the maths, and end up a bag or two short.

Round vs square holes

Most hand augers and post-hole diggers produce a roughly round hole, so the cylinder formula is the right one for most DIY jobs. If you're digging with a trencher or spade and end up with a squarer hole, the volume will be somewhat larger than the cylinder estimate for the same nominal "diameter" — square holes have more volume than a circle of the same width. When in doubt, round up your bag order rather than down.

Embedment Depth Recap (1/3 of Post Length)

As a general starting point, aim for roughly one-third of the post's total length to be embedded in the ground. A 1.8m post, for example, would typically have around 600mm below ground and 1.2m exposed above.

This is a rule of thumb, not a fixed engineering figure — actual embedment depth depends on:

• Wall height and the load on the post. Taller retaining walls push more soil load onto each post, which generally calls for deeper embedment.
• Soil type. Loose, sandy, or previously disturbed soil holds a post less firmly than compacted clay, so deeper or wider footings compensate for the weaker ground.
• Site slope. On sloping ground, embedment depth is usually measured from the lower (downhill) ground level, not the point where the post enters the soil on the uphill side.
• Council or engineered specifications. If your wall required approval, the embedment depth may be specified in the approved drawings and should override the general rule of thumb.

If you're working from an engineer's spec or council-approved drawings, always follow the documented embedment depth over the general one-third guide — those figures account for your specific soil and wall height in a way a rule of thumb can't.

Worked Example

Let's work through a typical residential post:

• Post length: 1.8m total
• Embedment depth (1/3 rule): 600mm in the ground
• Hole diameter: 300mm

Using the cylinder formula for a 300mm diameter, 600mm deep hole gives roughly 42 litres of gross hole volume. Now subtract the space the post itself displaces. For a typical 90mm x 90mm timber post or a similarly sized steel post over 600mm of embedment, that's roughly 5 litres of displaced volume — leaving around 37 litres of actual concrete needed.

Since one 20kg bag of concrete makes roughly 10 litres once mixed, that works out to:

37 litres ÷ 10 litres per bag ≈ 3.7 bags → round up to 4 bags

Always round up rather than down. A part-bag shortfall mid-pour means either an awkward top-up with a different batch of mixed concrete (which can create a weak seam) or a delay while you get more — both worth avoiding for the cost of one extra bag.

Breaking down where the numbers come from

It's worth understanding each step of that calculation rather than just taking the final bag count on faith, since your own hole will rarely match the worked example exactly:

• Step 1 — gross hole volume. Measure your actual hole diameter and depth, then apply the cylinder formula (π × radius² × depth) or use the reference table above for a close estimate.
• Step 2 — subtract post displacement. Work out the cross-sectional area of your post and multiply by the embedded length. For a typical 90mm x 90mm timber post, that's roughly 8,100mm² of cross-section; for round steel posts, use the post diameter in the same cylinder formula at a smaller scale.
• Step 3 — convert to bags. Divide the net concrete volume (hole volume minus post displacement) by roughly 10 litres per 20kg bag.
• Step 4 — round up. Always round to the next whole bag, and add a small buffer if you're setting several posts and want to avoid running short on the last one.

A second example — a smaller fence-height post

For a shorter post — say a 1.2m post with 400mm embedment in a 250mm diameter hole — the gross hole volume is around 20 litres. Subtracting roughly 3 litres for the post itself leaves about 17 litres of concrete, or:

17 litres ÷ 10 litres per bag ≈ 1.7 bags → round up to 2 bags

This is a useful sanity check: smaller posts on shorter walls generally land in the 2-bag range, while taller posts on deeper footings push toward 3–4 bags each. If your estimate is coming out well outside this range, it's worth double-checking your hole measurements before ordering.

Buying in 20kg Bags — How Many Per Post

As a rough reference table, based on the cylinder method above and typical post displacement:

Hole size (diameter x depth)	Approx. bags needed (20kg)
250mm x 400mm	2 bags
250mm x 500mm	2–3 bags
300mm x 600mm	3–4 bags
350mm x 700mm	5–6 bags

These figures are a planning guide, not an exact substitute for measuring your own hole and post size — soil conditions, post cross-section, and how tightly you tamp the concrete all shift the number slightly. Use the table to get your order roughly right, then round up.

Common Australian post sizes and typical hole allowances

Post size influences both the hole diameter you'll dig and how much volume the post itself displaces:

• 65mm x 65mm timber posts — common on lighter fencing, typically paired with a 200–250mm diameter hole.
• 90mm x 90mm timber posts — a common size for retaining wall posts, usually paired with a 250–300mm diameter hole.
• 100mm+ steel posts (C-section or similar) — often used for taller or engineered retaining walls, typically needing a 300–350mm diameter hole to give the concrete collar enough mass around the post.
• Sleeper-style posts — larger cross-section again, generally needing the wider end of the hole diameter range and correspondingly more concrete per hole.

As a general pattern, larger or heavier-duty posts need a wider hole, not just because the post itself is bigger, but because a taller or heavier wall places more load on each footing — the concrete collar needs enough mass to resist that load rather than just filling space around the post.

Ordering for a multi-post wall

For a full retaining wall run, multiply your per-post bag estimate by the number of posts, then add a small buffer — typically an extra bag or two for the whole job — to cover minor variations in hole size, spillage, and the occasional hole that turns out slightly larger than planned due to soil collapse or an awkward dig. It's far cheaper to have a spare bag left over than to stop a pour partway through a post run.

Buying in 20kg bags rather than ordering premixed truck concrete also makes sense for most residential post jobs, since post footings rarely need the volume a concrete truck is set up to deliver, and bags let you mix exactly what each hole needs without waste.

Bags vs bulk — when to consider a different approach

Bagged concrete is the practical choice for the vast majority of post-setting jobs, since it lets you mix exactly what each hole needs on site without excess. It's really only worth considering bulk or truck-delivered concrete once you're setting a very large number of posts in one visit — enough that the labour of hand-mixing dozens of bags starts to outweigh the convenience and precision of bagged product. For a typical residential retaining wall of a dozen posts or fewer, bagged concrete remains the more practical option, and is what most of our trade and DIY customers order for this exact job.

Seasonal buffer

In cooler months, it's worth adding a small buffer to your bag order beyond the calculated minimum. Cold weather can make bagged concrete slightly harder to work into every corner of the hole, particularly around irregular post shapes, which can mean a touch more wastage than in warmer conditions. It's a minor adjustment, but worth factoring in if you're ordering right down to the calculated minimum rather than rounding generously.

Use the Material Estimator for your Exact Wall

The table and worked examples above cover the common cases, but every wall is a little different — post spacing, wall height, soil type and hole size all shift the final number. Rather than doing the maths by hand for every post, our retaining wall calculator works out concrete quantities based on your actual wall dimensions and post count, so your order matches the job rather than a generic estimate.

It's particularly useful if you're pricing up a full wall before starting, since it lets you compare bag counts and rough cost across different post spacings and hole sizes before you commit to digging. Running a few different post-spacing scenarios through the calculator before you start can also help you decide whether tighter post spacing (more posts, smaller footings each) or wider spacing (fewer, larger footings) works out more cost-effective for your particular wall.

Need posts to go with your concrete order? See our steel retaining wall posts range, or the full concrete for post footings range to place both in one order.

Mistakes to avoid when estimating quantity

• Forgetting to subtract post displacement. It's a small adjustment, but skipping it on a multi-post job compounds into a noticeable shortfall.
• Assuming a "standard" hole size fits every post. Taller posts and taller walls need deeper, sometimes wider holes — check before ordering, don't assume last job's numbers apply.
• Not accounting for soil collapse. Loose or sandy soil can widen a hole beyond what you dug as the sides crumble, increasing the actual volume needed.
• Rounding down to save a bag. The cost difference of one extra bag is minor compared to the hassle of stopping mid-pour to buy more.
• Mixing bags to inconsistent ratios. If you're mixing several bags for one large hole, keep the water ratio consistent across batches so the footing cures evenly.

How soil type changes your order

The quantity table above assumes a fairly standard hole in reasonably firm, stable soil. Real sites vary, and soil type is one of the biggest reasons actual usage differs from a calculated estimate:

• Clay soils tend to hold a dug hole's shape well, so your actual volume should track close to the calculated cylinder volume.
• Sandy or loose soils often collapse slightly at the edges as you dig, widening the effective hole beyond what you measured — budget for a bit more concrete than the calculation suggests.
• Rocky ground can produce an irregular hole shape that doesn't match the cylinder assumption well in either direction — dig first, then measure the actual hole before finalising your bag order where possible.
• Previously disturbed or backfilled soil — common near existing structures or recently landscaped areas — is often looser than undisturbed ground, and may need a wider hole for adequate holding strength regardless of what the concrete volume calculation alone suggests.

If you're not sure what you're dealing with until the auger goes in, it's sensible to order slightly more than the calculated minimum and treat the table above as a starting point rather than a guarantee, particularly on sites where you haven't dug before and don't know what's below the topsoil layer.

Estimating cost alongside quantity

Once you know your bag count, working out a rough budget is straightforward: multiply the number of bags by the per-bag price of whichever product you're using — rapid set, post mix, or footing concrete — and add a small buffer bag or two per job. For a multi-post wall, it's worth pricing this out before you start digging, since the concrete cost across a dozen or more posts is a meaningful line item in the overall build budget, not an afterthought once the posts are already in the ground.

If you're comparing rapid set against a standard mix for cost purposes, remember the trade-off isn't about strength — see our comparison guide for how the cost and cure time stack up against each other.

Double-checking before you order

Before finalising a bag order, it's worth running through a short checklist:

• Have you measured the actual hole diameter, not just estimated it by eye?
• Does your embedment depth match the one-third rule, or does your site call for something deeper?
• Have you accounted for post displacement, even roughly?
• Have you rounded up to the next whole bag, with a small buffer for a multi-post job?
• Does your soil type suggest you should budget a bit more than the calculated minimum?

Running through these five points takes a few minutes and avoids the far more time-consuming problem of a stalled pour partway through a post run, or discovering after the fact that a footing was short-poured and needs to be dug out and redone.